The letter by Haack et al. (2014) provides important information confirming the role of the thiamine transporter hTHTR2 in Leigh syndrome and the beneficial effect of biotin and/or thiamine treatment for patients harbouring mutations in the gene encoding hTHTR2, SLC19A3. In recent years many new pathogenic mutations have been reported in SLC19A3 resulting in several age-related neurological phenotypes like biotin-responsive basal ganglia disease (BBGD), Wernicke-like encephalopathy and Leigh syndrome and various responses to biotin and/or thiamine (Zeng et al., 2005; Kono et al., 2009; Debs et al., 2010; Yamada et al., 2010). The exact mechanism by which biotin and/or thiamine treatment results in alleviation of the phenotype or prevents further disease progression is still unclear, but the current data challenge the initial hypothesis that biotin-induced overexpression is the central mechanism. Table 1 summarizes all pathogenic mutations identified in SLC19A3 to date, the …